The intake and exhaust valves of an engine ideally are operated by cams and return springs to open at the right time and for a correct amount and duration to achieve the desired engine operation. Because the optimum cam profile for one engine condition, e.g. low speed, is much different than the optimum profile for another condition, e.g. high speed, various arrangements have been proposed to vary the valve opening as a function of the engine speed, load, or other parameter.
One such prior art proposal is to employ separate low lift and high lift cams with separate lifters, each lifter independently mounted in a carrier block and each acting on a different valve. In particular, two low lift tappets, one on either side of a single high lift tappet, independently operates each of two valves. The low lift tappets would always be operatively coupled to the valves while the high lift tappets would just idle during low speed operation. For high speed conditions, a locking mechanism couples the low and high lift tappets such that the action of the high lift cam overrides the low lift cams to control the low lift tappet and the valves for movement together. The locking mechanism comprises pins in the low lift tappets which must slide into holes in the high lift tappet thereby requiring tight tolerances in the three cooperating tappets. Further tight tolerance requirements apply to the cam carrier which must have accurately positioned bores for each of the three tappets.